The present invention relates to a process for separating and recovering solid metal catalysts from reaction mixtures in which they are dispersed, with cross-flow filtration using membrane separation devices.
Many metals, either in metallic form or in the form of compounds, are used as catalysts in numerous chemical processes. Since the catalyzed reaction as a rule takes place on the surface of the catalyst, the structure of the catalyst plays an essential part in its effectiveness. The metals used as catalysts are frequently quite expensive, and, moreover, must under no circumstances pass into effluents, for reasons of environmental protection. It is therefore necessary to remove the catalysts substantially quantitatively from the reaction mixture.
Catalysts have hitherto customarily been separated, using appropriate separation devices, by filtration, sedimentation or with the aid of separators in a so-called "dead end" filtration process, that is, the catalyst is compressed to a filter cake and is isolated in an essentially dry form. However, this process has a number of disadvantages. Firstly, gelatinous catalysts which are difficult to filter, such as, for example, nickel hydroxide sludges, can be filtered only with the aid of filtration auxiliaries. This means, however, that foreign substances are admixed with the catalyst, which, for this reason, cannot be re-used directly but must be regenerated, in processes which are frequently troublesome. Secondly, the structure of the very finely divided catalyst is frequently damaged as a result of the catalyst being compressed as a filter cake, so that it must likewise be regenerated before being re-used.